This invention relates generally to photopolymerizable compositions and more particularly to photopolymerizable compositions which include maleimide compounds as a component and methods of using the same.
Ethylenically unsaturated compounds, such as acrylate derivatives, can be polymerized by exposure to radiation, typically ultraviolet light, in the presence of a photoinitiating system. Typically, the photoinitiating system includes (1) a compound capable of initiating polymerization of the ethylenically unsaturated compound upon exposure to radiation (a xe2x80x9cphotoinitiatorxe2x80x9d) and optionally (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor. The coinitiators or synergists are typically alcohols, tertiary amines, amides, or ethers which have labile hydrogens attached to a carbon adjacent to a heteroatom. Currently commercially available photoinitiators include benzophenones and derivatives thereof, such as thioxanthone derivatives.
Maleimides, and in particular N-aliphatic and ortho-substituted (i.e. xe2x80x9ctwistedxe2x80x9d) N-aromatic maleimides have been investigated as comonomer-photoinitiators of UV-curable acrylic systems. These maleimides have been observed to require the presence of a hydrogen atom-donor synergist (such as an amine, ether, thiol, or the like) to obtain reasonable rates of initiation and polymerization.
J. Put and F. C. De Schryver, xe2x80x9cPhotochemistry of Nonconjugated Bichromophoric Systems,xe2x80x9d Journal of the American Chemical Society. 95, 1, 137-45 (1973); xe2x80x9cPhotocyloaddition Polymerization. I. Preparation and Characterization of Poly-N,Nxe2x80x2-polymethylenebisdichloromaleimides,xe2x80x9d Journal of Polymer Science: Part A-1. 8, 1939-48 (1970); and N. Boens et al., xe2x80x9cSolid-State Ultraviolet Irradiation of Some Maleimides and Bismaleimides,xe2x80x9d Journal of Polymer Science: Polymer Chemistry Edition 13, 210-13 (1975) report studies of the sensitization of maleimides with benzophenone and thioxanthones for the formation of maleimide 2+2 cycloadducts. F. C. De Schryver, et al, xe2x80x9cPhotochemistry of Nonconjugated Bichromophoric Systems. Photopolymerization of N,N-Alkylenebis (dimethylmaleimides),xe2x80x9d Journal of the American Chemical Society 96, 20, 6463-69 (1974) also report the formation polymer networks by the sensitized step-wise 2+2 cycloaddition of maleimides.
The present invention is directed to photopolymerizable compositions and methods for radiation curing of the same. The compositions of the invention include at least one radiation curable compound. The compositions preferably include ethylenically unsaturated compounds, and in particular, acrylate derivatives.
The compositions further include at least one maleimide which is capable of initiating the photopolymerization of a radiation curable compound. The maleimide can be an alkyl maleimide, functionalized aliphatic maleimide, aromatic maleimide, maleimide, maleic anhydride, or mixtures thereof. The maleimide component can be substantially completely consumed during initiation and photopolymerization (incorporated into the polymer structure).
The composition further includes additional photoactive compounds as sensitizers. Specifically, the composition includes at least one benzophenone compound which is capable of initiating polymerization of the ethylenically unsaturated compound upon exposure to radiation. As used herein, the term xe2x80x9cbenzophenone compoundxe2x80x9d includes benzophenone and derivatives thereof, including thioxanthone and derivatives of thioxanthone. In addition, the composition optionally includes at least one coinitiator or synergist as a hydrogen atom donor or an electron donor. The hydrogen atom donor can be present as a separate compound or can be included as a molecular component of the photopolymerizable compound. The synergistic effect described herein which occurs when a sensitizer is used in conjunction with a maleimide in the presence of a hydrogen atom donor has been observed to dramatically increase rates of polymerization of typical acrylic photo-curable systems, as compared to rates obtained using the same acrylic photo-curable system with either maleimide with a hydrogen-atom donor or the sensitizer with a hydrogen-atom donor as the initiating system.
The present invention also provides methods for radiation curing of photopolymerizable compositions of the invention.